Enhanced thermal properties of epoxy composites by constructing thermal conduction networks with low content of three-dimensional graphene.

Micro/nano electronic devices heat dissipation depends heavily on the thermal interface materials (TIMs). Despite notable progress, it is hard to efficaciously enhance the thermal properties of the hybrid TIMs with high-load additives due to an absence of effective heat transfer routes. Herein, the low content of three-dimensional (3D) graphene with interconnected networks is adopted as the additive to improve the thermal properties of epoxy composite TIMs. The thermal diffusivity and thermal conductivity of the as-prepared hybrids were dramatically improved by constructing thermal conduction networks after adding 3D graphene as fillers. The 3D graphene/epoxy hybrid's optimal thermal characteristics were observed at 1.5 wt% of 3D graphene content, corresponding to a maximum enhancement of 683%. Besides, heat transfer experiments were further performed to determine the superb heat dissipation potential of the 3D graphene/epoxy hybrids. Moreover, the 3D graphene/epoxy composite TIM was also applied to high-power LED to improve heat dissipation. It effectively reduced the maximum temperature from 79.8 °C to 74.3 °C. These results are beneficial for the better cooling performance of electronic devices and provide useful guidelines for advancing the next-generation TIMs.

Robust, Conductive, and High Loading Fiber-Shaped Electrodes Fabricated by 3D Active Coating for Flexible Energy Storage Devices.

Flexible power sources are critical to achieve the wide adoption of portable and wearable electronics. Herein, a facile and general strategy of fabricating a fibrous electrode was developed by 3D active coating technology, in which a stepping syringe with electrode paste was synchronously injected onto a rotating conductive wire, distinguished from the conventional direct-write 3D printing without a current collector. A series of such electrodes with different coating weight can be fabricated accurately and efficiently by adjusting critical process parameters following a set of derived equations. The demonstrated fibrous Zn-MnO2 battery with a high commercial ε-MnO2 loading of 14.9 mg cm-2 onto a stainless steel wire shows a reasonable energy density of 108 mWh cm-3, while the fiber-shaped supercapacitor with commercial porous graphene exhibits a high capacitance of 142.9 F g-1 and good durability for bending 10,000 cycles. This work constructs a bridge between materials and fiber-shaped electrodes for flexible energy storage devices.

Boosting Electrocatalytic Activity of Single Atom Catalysts Supported on Nitrogen-Doped Carbon through N Coordination Environment Engineering.

Nonprecious group metal (NPGM)-based single atom catalysts (SACs) hold a great potential in electrocatalysis and dopant engineering has been extensively exploited to boost their catalytic activity, while the coordination environment of dopant, which also significantly affects the electronic structure of SACs, and consequently their electrocatalytic performance, have been largely ignored. Here, by adopting a precursor modulation strategy, the authors successfully synthesize single cobalt atom catalysts embedded in nitrogen-doped carbon, Co-N/C, with similar overall Co and N concentrations but different N types, that is, pyridinic N (NP ), graphitic N (NG ), and pyrrolic N (NPY ). Co-N/C with the Co-N4 moieties coordinated with NG displays far superior activity for oxygen reduction (ORR) and evolution reactions, and superior activity and stability in both zinc-air batteries and proton exchange membrane fuel cells. Density functional theory calculation indicates that coordinated N species in particular NG functions as electron donors to the Co core of Co-N4 active sites, leading to the downshift of d-band center of Co-N4 and weakening the binding energies of the intermediates on Co-N4 sites, thus, significantly promoting catalytic kinetics and thermodynamics for ORR in a full pH range condition.

Graphene Nanosphere as Advanced Electrode Material to Promote High Performance Symmetrical Supercapacitor.

To get carbon electrode with both excellent gravimetric and volumetric capacitances at high mass loadings is critical to supercapacitors. Herein, cracked defective graphene nanospheres (GNS) well meet above requirements. The morphology and structure of the GNS are controlled by polystyrene sphere template/glucose ratio, microwave heating time, and Fe content. The typical GNS with specific surface area of 2794 m2 g-1 , pore volume of 1.48 cm3 g-1 , and packing density of 0.74 g cm-3 performs high gravimetric and volumetric capacitances of 529 F g-1 and 392 F cm-3 at 1A g-1 with a capacitance retention of 62.5% at 100 A g-1 in a three-electrode system in 6 mol L-1 KOH aqueous electrolyte. In a two-electrode system, the GNS possesses energy density of 18.6 Wh kg-1 (13.8 Wh L-1 ) at the power density of 504 W kg-1 , which is higher than all reported pure carbon materials in gravimetric energy density and higher than all reported heteroatom-doped carbon materials in volumetric energy density, in aqueous solution, as far as it is known. A structural feature of carbon materials that possess both high energy density and high power density is pointed out here.

Electricity generation from ionic solution flowing through packed three-dimensional graphene powders.

Herein, we demonstrate a distinctive energy harvesting method that electricity can be generated from the ionic solution flowing through the interstices between packed three-dimensional graphene powders. A constructed electrokinetic nanogenerator with an effective flow area of ∼0.34 cm2can generate a large current of 91.33 nA under 10-6M NaCl solution with a flow rate of 0.4 ml min-1, corresponding to a maximum power density of 0.45µW m-2. Besides, it shows a good linear relationship between the streaming current and the flow rate, suggesting that it could be used as a self-powered micro-flowmeter. These results provide a convenient way for clean energy harvesting and show a bright future for self-powered systems.

MnS@N,S Co-Doped Carbon Core/Shell Nanocubes: Sulfur-Bridged Bonds Enhanced Na-Storage Properties Revealed by In Situ Raman Spectroscopy and Transmission Electron Microscopy.

Rational structure and morphology design are of great significance to realize excellent Na storage for advanced electrode materials in sodium-ion batteries (SIBs). Herein, a cube-like core/shell composite of single MnS nanocubes (≈50 nm) encapsulated in N, S co-doped carbon (MnS@NSC) with strong CSMn bond interactions is successfully prepared as outstanding anode material for SIBs. The carbon shell significantly restricts the expansion of the MnS volume in successive sodiation/desodiation processes, as demonstrated by in situ transmission electron microscopy (TEM) of one single MnS@NSC nanocube. Moreover, the in situ generated CSMn bonds between the MnS core and carbon shell play a significant role in improving the Na-storage stability and reversibility of MnS@NSC, as revealed by in situ Raman and TEM. As a result, MnS@NSC exhibits a high reversible specific capacity of 594.2 mAh g-1 at a current density of 100 mA g-1 and an excellent rate performance. It also achieves a remarkable cycling stability of 329.1 mAh g-1 after 3000 charge/discharge cycles at 1 A g-1 corresponding to a low capacity attenuation rate of 0.0068% per cycle, which is superior to that of pristine MnS and most of the reported Mn-based anode materials in SIBs.

Molecular dynamics simulations of the thermal conductivity of graphene for application in wearable devices.

Graphene has been highlighted as a great potential material in wearable devices, owing to its extraordinary properties such as mechanical softness, high electrical conductivity and ultra-thin thickness. However, there are remaining challenges in thermal dissipation of graphene under such complicated conditions, which significantly affect the performance of portable electronics. Using molecular dynamics simulations, we have performed systematic analysis of thermal performance for the application in wearable devices in terms of graphene with defects, under uniaxial tensile strain, and vertical stress. Three kinds morphology of defects (horizontal line defect, circular defect, and vertical line defect) are constructed by deleting atoms on the pristine graphene plane. The thermal conductivity is related to the projected defected area perpendicular to the direction of the heat current. The relative thermal conductivity displays a cubic decreasing trend with the increase of uniaxial tensile strain. Besides, the thermal conductivity of graphene is not only related to the deformation quantity, but also related to the type of compression region. Our results show that the thermal conductivity decreases a lot under local stress with larger vertical deformation, while no obvious decline is observed under the global stress. This study aims to provide guidelines and ballpark estimates for experimentalists fabricating flexible devices from graphene.

Advances in the high performance polymer electrolyte membranes for fuel cells.

This critical review tersely and concisely reviews the recent development of the polymer electrolyte membranes and the relationship between their properties and affecting factors like operation temperature. In the first section, the advantages and shortcomings of the corresponding polymer electrolyte membrane fuel cells are analyzed. Then, the limitations of Nafion membranes and their alternatives to large-scale commercial applications are discussed. Secondly, the concepts and approaches of the alternative proton exchange membranes for low temperature and high temperature fuel cells are described. The highlights of the current scientific achievements are given for various aspects of approaches. Thirdly, the progress of anion exchange membranes is presented. Finally, the perspectives of future trends on polymer electrolyte membranes for different applications are commented on (400 references).

Li-S Chemistry of Manganese Phosphides Nanoparticles With Optimized Phase.

The targeted synthesis of manganese phosphides with target phase remains a huge challenge because of their various stoichiometries and phase-dependent physicochemical properties. In this study, phosphorus-rich MnP, manganese-rich Mn2 P, and their heterostructure MnP-Mn2 P nanoparticles evenly dispersed on porous carbon are accurately synthesized by a convenient one-pot heat treatment of phosphate resin combined with Mn2+ . Moreover, their electrochemical properties are systematically investigated as sulfur hosts in lithium-sulfur batteries. Density functional theory calculations demonstrate the superior adsorption, catalysis capabilities, and electrical conductivity of MnP-Mn2 P/C, compared with MnP/C and Mn2 P/C. The MnP-Mn2 P/C@S exhibits an excellent capacity of 763.3 mAh g-1 at 5 C with a capacity decay rate of only 0.013% after 2000 cycles. A phase evolution product (MnS) of MnP-Mn2 P/C@S is detected during the catalysis of MnP-Mn2 P/C with polysulfides redox through in situ X-ray diffraction and Raman spectroscopy. At a sulfur loading of up to 8 mg cm-2 , the MnP-Mn2 P/C@S achieves an area capacity of 6.4 mAh cm-2 at 0.2 C. A pouch cell with the MnP-Mn2 P/C@S cathode exhibits an initial energy density of 360 Wh kg-1 .

Bimetallic carbide nanocomposite enhanced Pt catalyst with high activity and stability for the oxygen reduction reaction.

Nanocomposites consisting of the bimetallic carbide Co(6)Mo(6)C(2) supported on graphitic carbon ((g)C) were synthesized in situ by an anion-exchange method for the first time. The Co(6)Mo(6)C(2)/(g)C nanocomposites were not only chemically stable but also electrochemically stable. The catalyst prepared by loading Pt nanoparticles onto Co(6)Mo(6)C(2)/(g)C was evaluated for the oxygen reduction reaction in acidic solution and showed superior activity and stability in comparison with commercial Pt/C. The higher mass activity of the Pt-Co(6)Mo(6)C(2)/(g)C catalyst indicated that less Pt would be required for the same performance, which in turn would reduce the cost of the fuel cell electrocatalyst. The method reported here will promote broader interest in the further development of other nanostructured materials for real-world applications.

Ion-exchange-assisted synthesis of Pt-VC nanoparticles loaded on graphitized carbon: a high-performance nanocomposite electrocatalyst for oxygen-reduction reactions.

Carbide-based electrocatalysts are superior to traditional carbon-based electrocatalysts, such as the commercial Pt/C electrocatalysts, in terms of their mass activity and stability. Herein, we report a general approach for the preparation of a nanocomposite electrocatalyst of platinum and vanadium carbide nanoparticles that are loaded onto graphitized carbon. The nanocomposite, which was prepared in a localized and controlled fashion by using an ion-exchange process, was an effective electrocatalyst for the oxygen-reduction reaction (ORR). Both the stability and the durability of the Pt-VC/GC nanocomposite catalyst could be enhanced compared with the state-of-the-art Pt/C. This approach can be extended to the synthesis of other metal-carbide-based nanocatalysts. Moreover, this straightforward synthesis of high-performance composite nanocatalysts can be scaled up to meet the requirements for mass production.

Nano-architectures of ordered hollow carbon spheres filled with carbon webs by template-free controllable synthesis.

Hollow carbon spheres with a carbon network structure inside have been synthesized in the absence of templates for the first time. The samples were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, N(2) adsorption-desorption, and thermogravimetric analyses. The experimental results indicate that the core and the shell of this new structure contain both mesopores and micropores. Furthermore, the inner space and thickness of the hollow carbon spheres can be controlled by adjusting the molar ratio of the glucose and Na(2)SnO(3)·H(2)O. Moreover, a possible formation mechanism has been suggested on the basis of time-dependent experiments. The electrocatalytic activity of methanol oxidation on Pt supported on HC electrocatalyst (Pt/HC@C) is 1.8 times higher than that of Pt supported on commercial Vulcan XC-72 carbon (Pt/C) electrocatalyst at the same Pt loadings.

Interface engineering of NiMoSx heterostructure nanorods for efficient oxygen evolution reaction.

The development of electrocatalyst with efficient and stability for water oxidation is the key to enhance the efficiency of water electrolysis. Interface engineering can modify the local electronic structure of active sites, which is one of the important strategy to enhance catalytic activity. Herein, we synthesized NiMoSx heterostructure nanorods by simple hydrothermal method. The self supporting electrode of NiMoSx heterostructure nanorods grown in situ on nickel foam can reduce the indirect contact resistance between the substrate and the catalyst, and promote the timely release of bubbles produced by the oxygen evolution reaction. The heterogeneous interface in NiMoSx can provide abundant electroactive centers and optimize the adsorption energy of active intermediates. NiMoSx heterostructure nanorods showed excellent oxygen evolution catalytic activity (η100 = 279 mV, η1000 = 436 mV, Tafel slope b = 72.3 mV dec-1) and more than 200 hours of sustainable durability in 1 M KOH. When NiMoSx heterostructure nanorods are used as anode materials for water electrolysis, the electrolytic cell could obtain 10 mA cm-2 at 1.48 V. The current research results not only show that NiMoSx nanostructure is an excellent oxygen evolution electrocatalyst, At the same time, it also provides a valuable interface regulation method for the design of high-performance heterostructure electrocatalyst.

Ru doping NiCoP hetero-nanowires with modulated electronic structure for efficient overall water splitting.

Herein, a novel Ru-doped bimetal phosphide (Ru-NiCoP) heterostructure electrocatalyst on Ni foam is successfully synthesized through a multi-step hydrothermal reaction, ion exchange, and phosphorization method for efficient overall water splitting in alkaline media. The doping of Ru and P can effectively optimize the electronic structure and expose more active sites. The unique 3D interconnected nanowires not only ensures the uniform distribution of Ru coupled with NiCoP, but also endows the Ru-NiCoP/NF with the large ECSA, the fast electron transport and the favorable reaction kinetics attributes. Benefiting from the compositional and structural advantages, Ru-NiCoP/NF catalyst exhibits significantly enhancedcatalytic activities along with excellent stability, only needing 32.3 mV at 10 mA cm-2 for HER and 233.8 mV at 50 mA cm-2 for OER. In particular, when Ru-NiCoP/NF is employed as both cathode and anode electrodes,a small voltage of 1.50 V is required to reach 30 mA cm-2for overall water splittingwith an impressive stability. This study provides an alternative strategyon the design and development of high performance catalysts foroverall water splittingand other energy conversion fields.

Enhanced oxygen reduction and methanol oxidation reaction over self-assembled Pt-M (M = Co, Ni) nanoflowers.

Herein, we introduce a facile approach to synthesize a unique class of Pt-M (M = Ni, Co) catalysts with a nanoflower structure for boosting both oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). By controlling the surface-active agents, we modified the functional groups surrounding the Pt atoms, tuned the alloying of Pt and the transition metals Ni and Co, and prepared two different kinds of nanodendrites. Their successful synthesis depends on the selection and amount of surfactants (hexadecyltrimethylammonium bromide (CTAB), Polyvinylpyrrolidone (PVP)). Besides, by controlling reaction time, we also explored the forming procedures for Pt-Co globularia nanodendrite (Pt-Co GND) and Pt-Ni petalody nanodendrite (Pt-Ni PND). Our investigation highlights the importance of complex nanoarchitecture, which enables surface and interface modification to achieve excellent catalytic performance in fuel cell electrocatalysis. The characterization of the as-prepared catalysts reveals a high electrochemical surface area and mass activity (2041 mAmgPt-1and 950 mAmgPt-1 for Pt-Co GND and Pt-Ni PND, respectively, for ORR). Furthermore, Pt-Co GND showed a high MOR activity, with a mass activity value recorded at 1615 mAmgPt-1 which is far superior to that for Pt/C. Moreover, both catalysts retain high activity after accelerated durability tests (ADTs). The electron transfer number was calculated by performing the rotating ring-disk electrode (RRDE) measurements. Due to abundant active sites of Pt, both Pt-Co GND and Pt-Ni PND exhibit a 4e- pathway for ORR with electron transfer number of >3.95.

In situ FTIR spectroelectrochemical study on the mechanism of ethylene glycol electrocatalytic oxidation at a Pd electrode.

The adsorption and electrooxidation pathways of ethylene glycol (EG) on polycrystalline palladium surfaces have been investigated in both alkaline and acidic media by in situ FTIR spectroscopy in conjunction with cyclic voltammetry. Palladium exhibits a high electrocatalytic activity in alkaline solution with low onset oxidation potentials and high current densities, depending on the pH, as well as on the supporting electrolyte. Higher potentials are required for EG oxidation in acidic solutions, where the catalytic performance decreases with increasing the pH. The products and intermediates of EG oxidation on Pd are influenced by the pH. In alkaline media, both C(2) species (glycolate, glyoxal, glyoxylate and oxalate) and C(1) species (formate and carbonate) are formed in mutual concentrations depending on the pH. In contrast, CO(2) is selectively produced in acidic aqueous solution.

A flexible and conductive MXene-coated fabric integrated with in situ sulfur loaded MXene nanosheets for long-life rechargeable Li-S batteries.

Lithium-sulfur (Li-S) batteries with high energy density, which show great application potential in flexible electronic products, have attracted a lot of research enthusiasm. However, the low utilization of sulfur and shuttle effect limit the application of Li-S batteries. Materials with a void structure and high conductivity can be used as a sulfur host to overcome these issues. Herein, a flexible MXene-coated textile fabric electrode (MF@Ti3C2Tx/S) is designed by integrating the MXene-coated textile fabric (MF) with in situ sulfur loaded MXene nanosheets (Ti3C2Tx/S). The MF provides a flexible 3D conductive framework, which is covered with Ti3C2Tx/S nanosheets to form the layer-by-layer structure. This unique structure not only provides enough space for volume expansion to maintain the structural stability in the electrochemical process, but also promotes the physical encapsulation and chemical adsorption of lithium polysulfides (LiPSs). Consequently, the MF@Ti3C2Tx/S50 electrode exhibits a high initial capacity of 916 mA h g-1 at 1C and an ultralong-term cycling stability of 674 mA h g-1 at 1C after 1000 cycles. Furthermore, this electrode also exhibits excellent rate performance at a high energy density (290 mA h g-1 at 5C after 800 cycles). A pouch cell is prepared by using the MF@Ti3C2Tx/S50 electrode and shows excellent cycle performances at different bending angles, which indicates that this study is valuable in the field of flexible energy storage. This work provides a new concept design for flexible Li-S batteries, which have great application potential as wearable and portable electronic devices.

Atomic Scale Mechanisms of Multimode Oxide Growth on Nickel-Chromium Alloy: Direct In Situ Observation of the Initial Oxide Nucleation and Growth.

The initial growth mode of oxide on alloy plays a decisive role in the development of protective oxide scales on metals and alloys, which is critical for their functionality for high temperature applications. However, the atomistic mechanisms dictating that the oxide growth remain elusive due to the lack of direct in situ observation of the initial oxide nucleation and growth at atomic-scale. Herein, we employed environmental transmission electron microscopy and the first-principles calculations to elucidate the initial atomic process of nickel-chromium (Ni-Cr) alloy oxidation. We directly revealed three different oxide growth modes of initial NiO islands on Ni-Cr alloy upon oxidation by O2, which result in distinct crystallography and morphology. The multimode oxide growth leads to irregular-shaped oxides, which is shown to be sensitive to the local mass transport. This localization of oxide growth mode is also demonstrated by the identified vigorous competence in oxide growth and thus shown to be kinetically controlled. The concept exemplified here provides insights into the oxide formation and has significant implications in other material and chemical processes involving oxygen gas, such as corrosion, heterogeneous catalysis, and ionic conduction.

High-capacity and high-rate Ni-Fe batteries based on mesostructured quaternary carbon/Fe/FeO/Fe3O4 hybrid material.

The Ni-Fe battery is a promising alternative to lithium ion batteries due to its long life, high reliability, and eco-friendly characteristics. However, passivation and self-discharge of the iron anode are the two main issues. Here, we demonstrate that controlling the valence state of the iron and coupling with carbon can solve these problems. We develop a mesostructured carbon/Fe/FeO/Fe3O4 hybrid by a one-step solid-state reaction. Experimental evidence reveals that the optimized system with three valence states of iron facilitates the redox kinetics, while the carbon layers can effectively enhance the charge transfer and suppress self-discharge. The hybrid anode exhibits high specific capacity of 604 mAh⋅g-1 at 1 A⋅g-1 and high cyclic stability. A Ni-Fe button battery is fabricated using the hybrid anode exhibits specific device energy of 127 Wh⋅kg-1 at a power density of 0.58 kW⋅kg-1 and maintains good capacity retention (90%) and coulombic efficiency (98.5%).

A facile strategy synthesized PtRhNi truncated triangle nanoflakes with PtRh-rich surface as highly active and stable bifunctional catalysts for direct methanol fuel cells.

Committed to improving the utilization efficiency of Pt atoms and accurately controlling the morphology and composition of nanocatalysts to boost the Pt-based catalyst performance has become the focus of research. Herein, the PtRhNi truncated triangular nanoflakes (TA-NFs) catalyst with a unique PtRh-rich surface structure was successfully prepared by an effective one-pot synthetic method based on the galvanic replace reaction. The freestanding 2D nanostructure of PtRhNi TA-NFs, intrinsically possessing much high specific surface area and surface atomic, and the PtRh-rich characteristics of the surface is undoubtedly the most feasible model to simultaneously achieve high atom utilization. Benefiting from this novel structure, the as-obtained PtRhNi TA-NFs nanocatalyst exhibits excellent performance for ORR and MOR, delivering a mass activity of 0.92 A mgpt-1 for ORR, which is 2.03, 1.64, and 6.9-fold higher than that of PtRhNi nanoparticls (NPs), PtNi truncated triangle nanoflakes (TA-NFs) and commercial Pt/C, respectively. In addition, after 20 k cycles ADT test, PtRhNi TA-NFs show only 10 mV negative shift of half-wave potential and retain 70% of initial value of mass activity. Furthermore, a mass activity is 1.28 A mgpt-1 is achieved after applying this unique nanocatalyst for MOR, which is 1.28,1.5, and 2.6 times higher than that of PtRhNi NPs, PtNi TA-NFs and Pt/C, respectively. Impressively, the PtRhNi TA-NFs nanocatalyst shows an ultrahigh stability even after 2 k cycles ADT measurement in acid solution, and the mass activity is only drop 2% of initial value. This work provides a new strategy to synthesis high-performance of bifunction Pt-based electrocatalyst for ORR and MOR fuel cells.